century to do so, as we shall see later.

Halley's book, the Synopsis Astronomiae Cometicae, is one of the most
valuable additions to astronomical literature since the time of Kepler.
He was first to attempt the calculation of the orbit of a comet, having
revived the ancient opinion that comets belong to the solar system,
moving in eccentric orbits round the sun, and his calculation of the
orbit of the comet of 1682 led him to predict correctly the return of
that comet in 1758. Halley's Study of Meteors.

Like other astronomers of his time he was greatly puzzled over the
well-known phenomena of shooting-stars, or meteors, making many
observations himself, and examining carefully the observations of other
astronomers. In 1714 he gave his views as to the origin and composition
of these mysterious visitors in the earth's atmosphere. As this
subject will be again referred to in a later chapter, Halley's views,
representing the most advanced views of his age, are of interest.

"The theory of the air seemeth at present," he says, "to be perfectly
well understood, and the differing densities thereof at all altitudes;
for supposing the same air to occupy spaces reciprocally proportional to
the quantity of the superior or incumbent air, I have elsewhere proved
that at forty miles high the air is rarer than at the surface of
the earth at three thousand times; and that the utmost height of the
atmosphere, which reflects light in the Crepusculum, is not fully
forty-five miles, notwithstanding which 'tis still manifest that some
sort of vapors, and those in no small quantity, arise nearly to that
height. An instance of this may be given in the great light the society
had an account of (vide Transact. Sep., 1676) from Dr. Wallis, which was
seen in very distant counties almost over all the south part of England.
Of which though the doctor could not get so particular a relation as was
requisite to determine the height thereof, yet from the distant places
it was seen in, it could not but be very many miles high.

"So likewise that meteor which was seen in 1708, on the 31st of July,
between nine and ten o'clock at night, was evidently between forty and
fifty miles perpendicularly high, and as near as I can gather, over
Shereness and the buoy on the Nore. For it was seen at London moving
horizontally from east by north to east by south at least fifty degrees
high, and at Redgrove, in Suffolk, on the Yarmouth road, about twenty
miles fr